Proprotein convertase subtilisin-kexin type 9 (hereinafter called “PCSK9”), also known as neural apoptosis-regulated convertase 1 (“NARC-1”), is a proteinase K-like subtilase identified as the 9th member of the secretory subtilase family; see Seidah et al., 2003 PNAS 100:928-933. The gene for PCSK9 localizes to human chromosome 1p33-p34.3; Seidah et al., supra. PCSK9 is expressed in cells capable of proliferation and differentiation including, for example, hepatocytes, kidney mesenchymal cells, intestinal ileum, and colon epithelia as well as embryonic brain telencephalon neurons; Seidah et al., supra.
Original synthesis of PCSK9 is in the form of an inactive enzyme precursor, or zymogen, of ˜72-kDa which undergoes autocatalytic, intramolecular processing in the endoplasmic reticulum (“ER”) to activate its functionality. This internal processing event has been reported to occur at the SSVFAQ↓SIPWNL158 motif rendering the first three N-terminal residues Ser-Ile-Pro (Benjannet et al., 2004 J. Biol. Chem. 279:48865-48875), and has been reported as a requirement of exit from the ER; Benjannet et al., supra; Seidah et al., supra. The cleaved protein is then secreted. The cleaved peptide remains associated with the activated and secreted enzyme; supra.
The gene sequence for human PCSK9, which is ˜22-kb long with 12 exons encoding a 692 amino acid protein, can be found, for example, at Deposit No. NP—777596.2. Human, mouse and rat PCSK9 nucleic acid sequences have been deposited; see, e.g., GenBank Accession Nos.: AX127530 (also AX207686), AX207688, and AX207690, respectively.
PCSK9 is disclosed and/or claimed in several patent publications including, but not limited to the following: PCT Publication Nos. WO 01/31007, WO 01/57081, WO 02/14358, WO 01/98468, WO 02/102993, WO 02/102994, WO 02/46383, WO 02/90526, WO 01/77137, and WO 01/34768; US Publication Nos. US 2004/0009553 and US 2003/0119038, and European Publication Nos. EP 1 440 981, EP 1 067 182, and EP 1 471 152.
PCSK9 has been ascribed a role in the differentiation of hepatic and neuronal cells (Seidah et al., supra.), is highly expressed in embryonic liver, and has been strongly implicated in cholesterol homeostasis. Recent studies seem to suggest a specific role in cholesterol biosynthesis or uptake. In a study of cholesterol-fed rats, Maxwell et al. found that PCSK9 was downregulated in a similar manner as three other genes involved in cholesterol biosynthesis, Maxwell et al., 2003 J. Lipid Res. 44:2109-2119. Interestingly, as well, the expression of PCSK9 was regulated by sterol regulatory element-binding proteins (“SREBP”), as seen with other genes involved in cholesterol metabolism; supra. These findings were later supported by a study of PCSK9 transcriptional regulation which demonstrated that such regulation was quite typical of other genes implicated in lipoprotein metabolism; Dubuc et al., 2004 Arterioscler. Thromb. Vasc. Biol. 24:1454-1459. PCSK9 expression was upregulated by statins in a manner attributed to the cholesterol-lowering effects of the drugs; supra. More, the PCSK9 promoters possessed two conserved sites involved in cholesterol regulation, a sterol regulatory element and an Sp1 site; supra. Adenoviral expression of PCSK9 has been shown to lead to a notable time-dependent increase in circulating LDL (Benjannet et al., 2004 J. Biol. Chem. 279:48865-48875). More, mice deleted of the PCSK9 gene have increased levels of hepatic LDL receptors and more rapidly clear LDL from the plasma; Rashid et al., 2005 Proc. Natl. Acad. Sci. USA 102:5374-5379. Recently it was reported that medium from HepG2 cells transiently transfected with PCSK9 reduced the amount of cell surface LDLR and internalization of LDL when transferred to untransfected HepG2 cells; see Cameron et al., 2006 Human Mol. Genet. 15:1551-1558. It was concluded that either PCSK9 or a factor acted upon by PCSK9 is secreted and is capable of degrading LDLR both in transfected and untransfected cells. More recently, it was demonstrated that purified PCSK9 added to the medium of HepG2 cells had the effect of reducing the number of cell-surface LDLRs in a dose- and time-dependent manner; Lagace et al., 2006 J. Clin. Invest. 116:2995-3005.
A number of mutations in the gene PCSK9 have also been conclusively associated with autosomal dominant hypercholesterolemia (“ADH”), an inherited metabolism disorder characterized by marked elevations of low density lipoprotein (“LDL”) particles in the plasma which can lead to premature cardiovascular failure; see Abifadel et al., 2003 Nature Genetics 34:154-156; Timms et al., 2004 Hum. Genet. 114:349-353; Leren, 2004 Clin. Genet. 65:419-422. A later-published study on the S127R mutation of Abifadel et al., supra, reported that patients carrying such a mutation exhibited higher total cholesterol and apoB100 in the plasma attributed to (1) an overproduction of apoB100-containing lipoproteins, such as low density lipoprotein (“LDL”), very low density lipoprotein (“VLDL”) and intermediate density lipoprotein (“IDL”), and (2) an associated reduction in clearance or conversion of said lipoproteins; Ouguerram et al., 2004 Arterioscler. Thromb. Vasc. Biol. 24:1448-1453.
Together, the studies referenced above evidence the fact that PCSK9 plays a role in the regulation of LDL production. Expression or upregulation of PCSK9 is associated with increased plasma levels of LDL cholesterol, and inhibition or the lack of expression of PCSK9 is associated with low LDL cholesterol plasma levels. Significantly, lower levels of LDL cholesterol associated with sequence variations in PCSK9 have conferred protection against coronary heart disease; Cohen, 2006 N. Engl. J. Med. 354:1264-1272
The identification of compounds and/or agents effective in the treatment of cardiovascular affliction is highly desirable. Reductions in LDL cholesterol levels have already demonstrated in clinical trials to be directly related to the rate of coronary events; Law et al., 2003 BMJ 326:1423-1427. More, recently moderate lifelong reduction in plasma LDL cholesterol levels has been shown to be substantially correlated with a substantial reduction in the incidence of coronary events; Cohen et al., supra. This was found to be the case even in populations with a high prevalence of non-lipid-related cardiovascular risk factors; supra. Accordingly, there is great benefit to be reaped from the managed control of LDL cholesterol levels.
Accordingly, it would be of great import to produce a therapeutic-based antagonist of PCSK9 that inhibits or antagonizes the activity of PCSK9 and the corresponding role PCSK9 plays in various therapeutic conditions.